(CNN) — At least three students are in critical condition after a shooting at Evergreen High School in Evergreen, Colorado, where more than 100 law enforcement officers are working to clear the school “room by room” as it remains on lockdown, officials said.

No suspects are in custody and and authorities are working to determine whether there was one or multiple shooters involved, said Jacki Kelley, public information officer for the sheriff’s department.

The first 911 call reporting an active shooter came in at 12:24 p.m. local time, said Kelley. The shooting took place on school grounds, but authorities could not clarify whether it happened inside or outside the building, she added.

The three minors with gunshot wounds are being treated at nearby St. Anthony’s Hospital and officials

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